Blasphemy! (I'm not a fan of sonic, except for their drinks).
Overrated, although better than Whataburger, but I don't think you can compare the two. Whataburger = fast food. Whataburger is the equivalent of McD's, Wendy's, Jack in the Box, etc. And I think Whataburger is the best fast-food burger.
Other burger joints (Sam's, Chris Madrid's, Chester's, Cheesy Jane's) are in a class by themselves.
The best burger I've had in San Antonio was $6.85, and from the Magnolia Pancake Haus. The secret? Super-sweet buns. They are awesome, homemade buns, with ground Angus, and fresh toppings.
http://www.magnoliapancakehaus.com/
I urge any burger fan to check it out and let me know what you think.